Fluidised bed heater

ABSTRACT

A heater comprising a bed of refractory particles within which a gaseous fuel and air are burned, the bed then being fluidised. The fuel and air are mixed prior to feeding through a passageway to inlet means through which the gases are admitted to the bed. Cooling means is provided to cool the gaseous mixture flowing through the passageway and inlet means to avoid combustion taking place upstream of the bed. The bed may be enclosed laterally by a hollow wall structure through which air can be blown to extract heat from the bed.

United States atet [1 1 Virr [451 May 20, 1975 1 FLUIDISED BED HEATER Michael John Virr, Stourbridge, England [75] Inventor:

[73] Assignee: Fluidfire Development Limited,

Dudley, England [22] Filed: July 27, 1973 [21] Appl. No.: 383,248

[30] Foreign Application Priority Data July 29, 1972 United Kingdom 35561/72 [52] US. Cl 431/170; 110/28 J; 431/160; 432/58 [51] Int. Cl. F23d 19/02 [58] Field of Search 431/7, 170, 160; 432/58, 432/15; 110/28 J; 122/4 D; 23/277 R [56] References Cited UNITED STATES, PATENTS Wetherbee 431/7 2,362,972 11/1944 Brownback 431/170 X 2,408,282 9/1946 Wolf 431/170 X 3,799,747 3/1974 Schmalfeld et al. 110/28 .1 X

Primary Examiner-Edward G. Favors Attorney, Agent, or Firml-lo1man & Stern [57] ABSTRACT A heater comprising a bed of refractory particles within which a gaseous fuel and air are burned, the bed then being fluidised. The fuel and air are mixed prior to feeding through a passageway to inlet means through which the gases are admitted to the bed. Cooling means is provided to cool the gaseous mixture flowing through the passageway and inlet means to avoid combustion taking place upstream of the bed. The bed may be enclosed laterally by a hollow wall structure through which air can be blown to extract heat from the bed.

10 Claims, 6 Drawing Figures PATENTED nmzmsvs 3,884,617

SHEET 10F 3 l 14 I6 17 22 1s o 21 IFIGII FIG 3 FIGA.

FLUIDISED BED HEATER BACKGROUND TO THE INVENTION A heater of the kind specified has previously been proposed and in this known heater the gaseous fuel and air are fed separately into the bed through different parts of the inlet means and permitted to mix only within the bed. It has been found that with apparatus arranged to operate in this manner, thorough mixing and even distribution of the fuel and air within the bed are difficult to achieve. If thorough mixing and even distribution are not achieved, the heater does not operate efficiently since combustion is not completed within the bed or within a part of the bed in which it is required to be completed, or expedients such as supplying a large excess of air must be adopted to ensure complete combustion.

In an attempt to overcome the foregoing difficulties, the previous proposal includes a deep bed. However, the deep bed itself provides a considerable resistance to the passage of the gases and it is necessary to expend a significant amount of energy on causing the gases to flow through the bed at a sufficiently high rate to fluidise the bed.

It is an object of the present invention to provide a heater of the kind specified wherein one or more of the foregoing disadvantages is reduced or avoided.

SUMMARY OF THE INVENTION According to the present invention there is provided a heater of the kind specified comprising means for mixing a gaseous fuel with air outside the bed and for feeding the mixture of fuel and air into the bed through the inlet means.

Preferably the feed means includes a passageway through which the mixture of fuel and air passes to the inlet means, and there being further provided cooling means for cooling the gaseous mixture which flows through the passageway and the inlet means.

With this arrangement, an homogeneous mixture of fuel and air can be fed to the bed and complete combustion of the fuel can thereby more easily be ensured. Furthermore. the heater can conveniently be so arranged that combustion takes place substantially entirely within a zone of the bed close to the inlet means. Furthermore, satisfactory operation can be achieved with a shallow bed, say a bed having a depth not greater than l2 inches. When a shallow bed is used, a correspondingly small expenditure of energy is required to cause the gases to flow through the bed at a speed such that the bed is fluidised.

During operation of heaters of the kind specified it is important to avoid combustion of the fuel taking place within or upstream of the inlet means, since this may damage same by overheating and may give rise to risk of injury to personnel or damage to equipment by explosion or otherwise.

With this arrangement, it is not essential for the inlet means to provide a resistance to flow of heat such that the inlet means alone avoids all risk of combustion taking place upstream of the inlet means.

The cooling means may comprise fins provided on the outside ofa wall of the passageway and a blower for causing air to flow over the fins, said wall being formed of thermally conductive material.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described, by way of example, with reference to the accompanying drawings wherein:

FIG. 1 is a vertical cross-sectional view of a heater of the kind specified intended for the heat treatment of workpieces in batches;

FIG. 2 is a longitudinal cross-sectional view of a further heater of the kind specified intended for the continuous heat treatment of endless members;

FIG. 3 is a fragmentary cross-sectional view on the line IIIIII of FIG. 2 and on an enlarged scale relative to FIG. 2;

FIG. 4 is a fragmentary cross-sectional view illustrating a modification of the apparatus shown in FIG. 2, FIG. 4 also being on an enlarged scale relative to FIG.

FIG, 5 is a fragmentary sectional view on the line VV of FIG. 4; and

FIG. 6 is a view similar to FIG. 1 illustrating a modification of the heater of FIG. 1.

DETAILED DESCRIPTION The heater shown in FIG. 1 comprises a treatment chamber 10 which contains a bed 11 of refractory particles such as sand. At the bottom of the chamber 10 there is an inlet means 12 which is adapted to admit gaseous fluid to the bed and is also adapted to support the particles of the bed within the'treatment chamber. In the particular example shown in FIG. I, the inlet means is in the form of a porous ceramic tile.

Within the bed 11 and just above the tile 12 there is provided a grid 13 which prevents workpieces being inserted into a zone of the bed immediately adjacent to the tile and in which zone combustion of the fuel occurs. It will be understood that free oxygen will be present in the combustion zone and that scale could be formed on workpieces if they were inserted into such zone. The grid 13 also prevents the tile 12 being dam aged by any workpiece which may be dropped in the bed. The tile also assists proper fluidisation of the bed by breaking up any large bubbles of gas which may form adjacent to the tile.

There is further provided feed means for feeding a mixture of a gaseous fuel and air through the tile 12 into the bed 11 to fluidise the latter and burntherein. The feed means includes a passageway 14 defined between a lower face of the tile 12 and a horizontal wall 15. A feed opening 16 is formed in the wall 15 below the centre of the tile 12 and a feed duct 17 communi cates with the passageway 14 through this opening, The wall 15 is spaced from the underside of the tile 12 just sufficiently to permit substantially unimpeded flow of the gases from the feed opening 16 to all parts of the tile 12. In this way, the velocity of the gases within the passageway 14 is maintained at as high a value as is reasonably practicable.

The feed means further includes a fuel/air mixing device 18 which may be of known construction and is connected with the feed duct 17. A blower is provided for feeding air to the device 18 and there is also provided an appropriate valve 19 for controlling the rate of flow of air to the device 18 so that the overall rate of flow through the feed duct 17 can be varied and the relative proportions of fuel and air can be varied by ad justment of the bleed screw in the device 18. The feed means would be arranged to maintain a preselected proportion of fuel in the gaseous mixture passing to the tile 12. Normally, a small excess of fuel would be provided in order to maintain a reducing atmosphere within the treatment chamber 10. The rate of supply of fuel and air may be controlled automatically to maintain a predetermined temperature within the treatment chamber.

The apparatus further comprises cooling means for cooling the wall across which the mixture of gaseous fuel and air passes before flowing through the tile 12 into the treatment chamber. The wall 15 is formed of metal and is therefore a relatively good conductor of heat. The cooling means comprises a chamber or duct having an upper boundary of which the wall 15 forms a part. The cooling means further comprises a fan 21 which communicates with the chamber 20 and is arranged to draw air therethrough from vents 22 provided at the periphery of the chamber-20, or alternatively to blow air into the chamber from which the a cooling air will escape throughthe vents 22. The air which traverses the chamber 20 extracts heat from the wall 15 and from that part of the feed duct 17 which extends through the chamber.

The periphery of the treatment chamber 10 is defined by a muffle 23 which is formed of a metal and is therefore a relatively good conductor of heat. A cylindrical wall 24 is spaced somewhat radially outwardly from the muffle 23 and is itself surrounded by a layer 25 of thermal insulating material. The top of the treatment chamber is open and communicates directly with an exhaust zone 26 defined by an upwardly diverging wall 27. The exhaust zone 26 is partly closed at its upper side by a lid 28 containing a body 29 of thermally conductive'material and formed with a central outlet opening 30 through which the products of combustion pass to a flue during operation of the heater. The lid 28 can be opened to permit workpieces to be inserted into and removed from the treatment chamber.

The annular space 31 between the muffle 23 and the wall 24 is closed at its lower end and at its upper end communicates with the lower part of the exhaust zone 26 through openings formed in the divergent wall 27 near to the lower end thereof. The apparatus comprises a further fan 32, the outlet of which communicates with the annular space 31 near to the lower end thereof. The fan can thus be used to blow cooling air through the annular space 31 into the exhaust zone 26. The annular space may be packed with large aluminium oxide balls or some other thermally conductive packing to increase the rate of transfer of heat from the muffle 23 to the air flowing through the annular space.

The fan 32 is intended to be used to blow cooling air through the annular space 31 when the heating load on the bed 11 is small, for example when workpieces therein have been raised to a temperature at which they are to be maintained for a period. It will be understood that, if the reducing atmosphere is to be maintained within the treatment chamber 10, the rate of supply of fuel to the bed 11 cannot be reduced without a corresponding reduction in the rate of supply of air. Whilst a reduction of the rate of supply of air and fuel would reduce the rate at which heat is released within the bed 4 11 by combustion, it is necessary to maintain the rate of flow of the gases through the bed above a minimum fluidising rate, if the bed is to be maintained in a fluid condition. It is therefore not always possible to reduce the rate of supply of fuel to the bed in accordance with a reduced heating load andto avoid the temperature of the bed rising above the required value, cooling air is blown through the annular space 31. The fan 31 may be controlled automatically by temperature-sensing means 33 which detects the temperature of the bed 11 and brings the fan 32 into operation when the temperature of the bed rises to a pre-set value. The control valves for the fuel and air supply are so arranged that the rate of supply of fuel and air cannot be reduced below the minimum fluidising rate unless the apparatus is shut down completely.

As previously mentioned, the fuel is supplied to the bed 11 at a rate slightly in excess of that rate at which the fuel can be completely oxidised by the air also supplied to the bed. Accordingly, the products of combustion which pass into the exhaust zone 26 contain combustible substances such as carbon monoxide. When the fan 32 is operating, the heated air which emerges from the annular space 31 into the exhaust zone mixes with the products of combustion and completes oxidation thereof so that no combustible gases emerge from the outlet 30.

If required, fins may be provided on the wall 15 to project downwardly therefrom into the cooling chamber 20. Such fins would increase the rate at which heat can be removed from the wall 15 by the cooling air. Since the wall 15 is maintained relatively cool, the gaseous mixture within the passageway 14 is also maintained relatively cool and the risk of combustion occurring within the passageway 14 is avoided.

In order to increase the rate at which the gases flow through unit area of the tile 12, the diameter of the latter may be reduced, as shown in FIG. 6 to a value smaller than the general diameter of the treatment chamber 10, the latter having an upwardly divergent portion at its lower end. In this way, the rate of gas flow per unit area of the tile 12 can be increased, thereby reducing the temperature of the tile and reducing the risk of combustion occurring within or below the tile.

In FIG. 6, parts corresponding to those hereinbefore described with reference to FIG. 1 are indicated by like reference numerals with the prefix 6 and the preceding description is deemed to apply'except for the differences hereinbefore mentioned.

The depth of the bed 11 is typically 1 foot when fluidised and 6 to 8 inches when slumped. The diameter of the bed is also 12 inches. It will be noted that the depth of the fluidised bed is not substantially greater than its diameter, and the depth of the slumped bed is substantially less than its diameter. The depth of the passageway 14 is typically three-eighths inch.

The velocity of the exhaust gases flowing upwardly from the bed decreases as these gases flow into the upwardly divergent exhaust zone 26, so that any particles carried upwardly from the bed will return thereto and will not be carried to the outlet 30. Workpieces which are to be heated in the apparatus are immersed in the comprises a treatment chamber 110 which contains a bed 111 of refractory particles, for example sand. Typically, the bed is 6 metres long, 0.6 metres wide and 0.3 metres deep. The sides of the treatment chambers are formed of stainless steel sheet and are enclosed externally by suitable thermal insulation. Rollers 110a or other guide means are provided at opposite ends of the bed to guide wires into and out of same. The furnace may be provided with an insulated hood to collect the hot gases off the fluidised bed. This hood being provided with a narrow slit to pass the wire through after attaching the coils at either end of the furnace. In a further development the hot gas is being passed to a lower temperature fluidised bed by means of a hot gas fan. This bed being typically 500C to obtain a treatment known as patenting.

At the bottom of the treatment chamber 110 there is provided inlet means for admitting gaseous fuel and air to the bed 111. This inlet means comprises a plurality of porous ceramic tiles 112. Typically, the tiles are arranged singularly along the length of the bed, there being 14 tiles along the whole length. The tiles are supported on metal support bars 113 arranged beneath marginal portions of the tiles and a deformable packing 114 is provided between adjacent edges of the tiles and between the outermost edges of the tiles and the walls of the treatment chamber. The pores of the tiles 112 are smaller than the particles of which the bed 111 is composed, so that the bed is supported on the tiles.

The apparatus further includes feed means for feeding a mixture of gaseous fuel and air into the bed 111 through the tiles 112. This feed means includes a passageway 115 which extends immediately beneath each of the tiles 112 and is defined between the lower face of such tile and a horizontal wall 116 which is formed of metal. Feed ducts 117 communicate with the passageway 115 through feed openings formed in the wall 116.

The feed means further includes a fan 118 which draws into the furnace air for combustion of the gaseous fuel. This air passes from the fan through a fuel/air proportioning valve 119 to which a gaseous fuel is also supplied. The valve 119 would normally be adjusted to provide a reducing atmosphere in the treatment chamber 110, typically an atmosphere containing 8 percent carbonmonoxide. From the valve 119, the air and fuel flow along respective feed ducts 120, 121 to a plurality of mixing devices, each of which is connected with one of the feed ducts 117 and is adapted to introduce a mixture of fuel and valves into such feed duct. As shown in FIG. 2, valve are provided in the various air and fuel feed ducts to enable the relative flow rates to different parts of the bed to be adjusted.

When the furnace is started from cold, the mixture of gaseous fuel and air is fed into the bed 111 from the passageway 115 and flows upwardly to the top of the bedv Pilot burners 122 situated at the open top of the treatment chamber 110 direct flames downwardly towards the bed and the gaseous mixture which reaches the surface of the bed is therefore ignited. Initially, combustion occurs in a zone immediately above the surface of the bed but as the particles of which the bed is composed become heated, combustion spreads downwardly into the bed and when the normal operating temperature is reached combustion rakes place entirely within the bed.

In order to eliminate the risk of the mixture of gase ous fuel and air burning upstream of the tiles 112, e001.-

ing means is provided for cooling the horizontal wall 116 and the gaseous mixture flowing through the passageway 115. This cooling means comprises fins 123 which project downwardly from the wall 116 into a cooling chamber 124, the upper boundary of which is formed by the wall 116. One or more further fans (not shown) are provided for blowing cooling air through the chamber 124.

It will be noted that the support bars 113 are in thermal contact with the horizontal wall 116 and that the cooling air passing through the chamber 124 therefore carries away heat conducted downwardly by the support bars 113.

The walls of the furnace made of high temperature stainless steel will naturally reach a fluid bed temperature. In order to prevent this causing heat to travel down the walls into the support bars 113, the passageway 125, is air cooled with separate fans which are not shown. I

In FIGS. 4 and 5 there is illustrated a modification of the continuous heat treatment furnace shown in FIGS. 2 and 3. In this modification, the inlet means for admitting the gaseous mixture to the bed 211 of refractory particles is formed by a plurality of stainless steel plates 212 arranged end-to-end, each plate being pierced to form a number of slits 213 which are evenly distributed throughout the area of the plate. Typically, the width of the slits is within the range to 250 microns.

The downwardly presented face of each of the plates 212 forms oneboundary of a corresponding passageway 214, an opposite boundary of-which is constituted by an associated wall 215. Each of the plates 213 and walls 215 is rectangular in shape so that the passageways 214 are each rectangular in plan view. The wall 215 converges with the associated plate 212 in a direction from one end of the passageway to the other, so that the passageway is tapered, as viewed in side elevation.

At the wider end of each of the passageways 214, a horizontal feed pipe 216 is interposed between the plate 212 and wall 215 and is welded to each of these. The feed pipe is formed with a plurality of feed openings 217 which are spaced apart along a length of the pipe, i.e., in a direction laterally of the passageway 214 and of the furnace. These feed openings provide for communication between the interior of the feed pipe and the associated passageway 14. Adjacent to the narrower end of the passageway, the plate 212 and the wall 215 are welded to each other and to the feed pipe 216 of an adjacent passageway so that the passageways 214 are completely closed except for the feed openings 217 and the slits 213.

A mixture of a gaseous fuel and air is supplied to the feed pipes 216 by means as described with reference to FIGS. 2 and 3.

The continuous anealing furnace is intended to heat 2,500 kilograms of wire per hour to a selected temperature within the range 850 to 950C. When the modified furnace illustrated in FIG. 4 is operating at this rate, the speed of gas flow through the passageways 214 is 3.5 metres per second, or even higher. In consequence of the tapered shape of the passageways 214, the gas speed is substantially the same in all regions of each passageway. Since the flame speed in the gaseous mixture is in the region of 0.32 metres per second,

7 there is no risk of combustion spreading into the passageways 214 from the inlet plate 212 whilst the furnace is operating at its maximum output rate.

When a lower rate of heat output is required, for example when the number of wires passing through the bed or the speed at which the wires pass through the bed is reduced, the rate of supply of fuel and air would be reduced accordingly to prevent the temperature of the bed 211 rising. Such control may be effected automatically. The rate at which air and gaseous fuel are supplied can be reduced to a rate one quarter of the normal rate, so that the gas speed within the passageways 214 will be reduced to approximately 0.8 metres per second. Since this gas speed will be attained throughout the passageways 214, there is no risk of combustion spreading into the passageways from the inlet plate 212 even when the rate of supply of fuel and air is reduced so considerably.

Should the temperature of the bed rise when the rate of supply of fuel and air to the bed is only one quarter the normal rate, automatic control means closes a valve in the fuel supply duct 121 so that combustion is discontinued and the bed cools. The flow of air may be maintained to maintain the bed in a fluid state, although this would provide an oxidising atmosphere in the treatment chamber.

During normal operation when air and fuel are supplied at a rate near to equal to the intended maximum rate, the gases passing through the inlet plate 212 will extract heat therefrom so that the temperature of the inlet plate, at least at that face thereof which is presented towards the passageway 214, will be maintained below the ignition temperature of the gaseous mixture, this temperature being approximately 750C. Thus, notwithstanding that the metal plate 212 will conduct heat at a relatively high rate downwardly from the bed 211 towards the passageway 214, combustion of the fuel downstream of the inlet plate will be avoided.

If required, the metal plate 212 may be substituted by a porous ceramic tile which is a relatively poor conductor of heat.

To maintain the gaseous mixture within the passageway 214 at a fairly low temperature, cooling means is provided for extracting heat therefrom. This cooling means comprises a plurality of fins 218 which project downwardly from the wall 215 into a cooling chamber or passage 219 through which air or some other coolant is caused to flow.

In each of the embodiments hereinbefore described with reference to the accompanying drawings, the cooling means comprises a passage which lies below a passageway through which the mixture of gaseous fuel and air is supplied to the inlet means. Since the crosssectional area of this passageway is small, it is considered that this arrangement should enable heat to be removed from the passageway at a sufficiently high rate. However, the rate at which heat is removed from the passageway may be increased by modifying the cooling means to provide a duct through which a coolant is passed, the duct extending either through the passageway so as to be in direct thermal contact with the mixture of gaseous fuel and air passing therethrough, or in direct thermal contact with the inlet means so as to extract heat directly therefrom.

I claim:

I. In a heater comprising a bed of refractory particles, inlet means for admitting gases to the bed and feed means for feeding a gaseous fuel and air into the bed through the inlet means at a rate such as to fluidise the bed, the improvement wherein the feed means includes means for mixing a gaseous fuel and air and for feeding of mixture of fuel and air to the inlet means, the feed means further including a passageway communicating with said mixing means and with said inlet means, and cooling means comprising a coolant duct in thermal communication with the gaseous mixture which flows through the passageway and the inletmeans when the heater is in use, and means for causing a fluid coolant to flow through said coolant ducts, whereby heat can be extracted from said gaseous mixture.

2. The improvement according to claim 1 wherein a wall of said passageway is formed of thermally conductive material and said cooling means comprises fins provided on the outside of said wall and a blower for causing air to flow over the fins.

3. The improvement according to claim 1 wherein the inlet means is of plate-like form and is arranged substantially horizontally, a downwardly presented face of the inlet means forms a boundary of said passageway, the depth of the passageway measured perpendic ular to said downwardly presented face of the inlet means is small, as compared with the distance between opposite boundaries of said face of the inlet means, the passageway has a feed opening, the area of which is substantially smaller than the area of said downwardly presented face of the inlet means, and the mixing means communicates with the passageway through said feed opening, whereby the gaseous mixture is caused to flow through the passageway along a path which extends generally across said face of the inlet means.

4. The improvement according to claim 3 wherein the depth of said passageway varies over said face of the inlet means, being greater adjacent to the feed opening and smaller remote therefrom.

5. The improvement according to claim 4 wherein said face of the inlet means is rectangular, a plurality of feed openings is provided adjacent to one margin of said face, and the passageway tapers towards the opposite margin of said face.

6. In a heater comprising a bed of refractory particles, inlet means for admitting gases to the bed and feed means for feeding a gaseous fuel and air into the bed through the inlet means at a rate such as to fluidise the bed, the improvement wherein the feed means includes means for mixing a gaseous fuel and air and for feeding the mixture of fuel and air to the inlet means, the bed being enclosed at its underside by said inlet means, a peripheral wall being provided for enclosing the bed laterally, that face of the peripheral wall which is presented inwardly towards the bed being upwardly divergent and the area of the bed in a horizontal plane near to its surface being substantially greater than the area of the inlet means in a horizontal plane.

7. In a heater comprising a bed of refractory particles, inlet means for admitting gases to the bed and feed means for feeding a gaseous fuel and air into the bed through the inlet means at a rate such as to fluidise the bed, the improvement wherein the feed means includes means for'mixing a gaseous fuel and air and for feeding the mixture of fuel and air to the inlet means, a wall structure which defines the periphery of the bed and which further defines at least one space internal to the wall structure, and further cooling means for causing a aperture through which air can pass from said internal space into said discharge zone.

10. The improvement according to claim 7 further comprising control means for controlling the flow of coolant through said space, the control means including means for determining the temperature of the bed and means for initiating said flow automatically when the temperature of the bed rises above a predetermined value. 

1. In a heater comprising a bed of refractory particles, inlet means for admitting gases to the bed and feed means for feeding a gaseous fuel and air into the bed through the inlet means at a rate such as to fluidise the bed, the improvement wherein the feed means includes means for mixing a gaseous fuel and air and for feeding of mixture of fuel and air to the inlet means, the feed means further including a passageway communicating with said mixing means and with said inlet means, and cooling means comprising a coolant duct in thermal communication with the gaseous mixture which flows through the passageway and the inlet means when the heater is in use, and means for causing a fluid coolant to flow through said coolant ducts, whereby heat can be extracted from said gaseous mixture.
 2. The improvement according to claim 1 wherein a wall of said passageway is formed of thermally conductive material and said cooling means comprises fins provided on the outside of said wall and a blower for causing air to flow over the fins.
 3. The improvement according to claim 1 wherein the inlet means is of plate-like form and is arranged substantially horizontally, a downwardly presented face of the inlet means forms a boundary of said passageway, the depth of the passageway measured perpendicular to said downwardly presented face of the inlet means is small, as compared with the distance between opposite boundaries of said face of the inlet means, the passageway has a feed opening, the area of which is substantially smaller than the area of said downwardly presented face of the inlet means, and the mixing means communicates with the passageway through said feed opening, whereby the gaseous mixture is caused to flow through the passageway along a path which extends generally across said face of the inlet means.
 4. The improvement according to claim 3 wherein the depth of said passageway varies over said face of the inlet means, being greater adjacent to the feed opening and smaller remote therefrom.
 5. The improvement according to claim 4 wherein said face of the inlet means is rectangular, a plurality of feed openings is provided adjacent to one margin of said face, and the passageway tapers towards the opposite margin of said face.
 6. In a heater comprising a bed of refractory particles, inlet means for admitting gases to the bed and feed means for feeding a gaseous fuel and air into the bed through the inlet means at a rate such as to fluidise the bed, the improvement wherein the feed means includes means for mixing a gaseous fuel and air and for feeding the mixture of fuel and air to the inlet means, the bed being enclosed at its underside by said inlet means, a peripheral wall being provided for enclosing the bed laterally, that face of the peripheral wall which is presented inwardly towards the bed being upwardly divergent and the area of the bed in a horizontal plane near to its surface being substantially greater than the area of the inlet means in a horizontal plane.
 7. In a heater comprising a bed of refractory particles, inlet means for admitting gases to the bed and feed means for feeding a gaseous fuel and air into the bed through the inlet means at a rate such as to fluidise the bed, the improvement wherein the feed meAns includes means for mixing a gaseous fuel and air and for feeding the mixture of fuel and air to the inlet means, a wall structure which defines the periphery of the bed and which further defines at least one space internal to the wall structure, and further cooling means for causing a coolant to pass through said space to remove from the wall structure heat imparted thereto by the bed.
 8. The improvement according to claim 7 wherein said wall structure comprises spaced inner and outer walls between which said space is defined.
 9. The improvement according to claim 7 wherein said further cooling means comprises a blower for causing air to flow into said space, the wall structure further defines an exhaust zone through which the products of combustion which leave the bed pass when the heater is operating and said wall structure further defines an aperture through which air can pass from said internal space into said discharge zone.
 10. The improvement according to claim 7 further comprising control means for controlling the flow of coolant through said space, the control means including means for determining the temperature of the bed and means for initiating said flow automatically when the temperature of the bed rises above a predetermined value. 